Scientists have found evidence that a colossal cliff stretching thousands of kilometres across ancient North America may have exposed the heart of the Grand Canyon nearly a billion years before it was carved by the Colorado River.
The study, led by the University of Southampton in the UK, proposes that a vast "great escarpment" of steep rocky cliffs formed when the supercontinent Rodinia broke apart 800 million years ago.
Experts suggest the kilometre-high cliffs extended along much of western North America, driving erosion of enormous volumes of rock and exposing ancient crystalline basement now visible in the Grand Canyon, southern Arizona.
Lead author Professor of Earth Science Thomas Gernon, from Southampton, said the Grand Canyon has a geological record spanning two billion years, but more than half of its rock record appears to be missing.
He added: "Our paper suggests the Canyon's basement rocks were progressively brought to the surface as part of an immense escarpment that developed during the breakup of an ancient supercontinent.
"The findings also shed light on the formation of the Great Unconformity, a mysterious gap in the rock record that spans over a billion years."
The study, published in Geology, included experts from the University of Southampton, GFZ Helmholtz Centre for Geosciences and University of Potsdam, both in Germany, and the University of Illinois Urbana-Champaign in the USA.
According to their findings, the ancient great escarpment would have stretched across what are now Arizona, Utah, Idaho, Wyoming, Colorado, Texas, Oklahoma, Arkansas, Missouri and Illinois.
This is where intense erosion ultimately exposed some of North America's oldest rocks, including most famously at the Grand Canyon, said Prof Gernon.
The researchers combined plate tectonic reconstructions with landscape evolution data to recreate how western North America evolved as Rodinia fragmented.
It showed the Grand Canyon occupied the same position relative to the ancient continental margin as many of today's great escarpments in South Africa and Brazil.
As the escarpment slowly retreated inland over tens of millions of years, it is predicted to have stripped away up to eight kilometres of rock in places.
The findings are consistent with evidence that the region experienced extraordinary erosion of some five to ten kilometres of rock long before the modern canyon formed.
"This long-lived tectonic landscape provides a missing piece in understanding why erosion associated with the Great Unconformity varies so dramatically across the southwestern US," said Prof Gernon.
He added: "Our work suggests that tectonic uplift related to continental rifting and breakup created both steep slopes and high ground, providing the mountainous terrain that rivers and glaciers could readily erode."
The study also reveals the ancient escarpment may have influenced far more than the Grand Canyon itself.
By creating a long-lived mountainous rim around western Laurentia - the ancient core of North America - it may have controlled where rivers flowed, sediments accumulated, and when rising seas flooded the continent prior to the so-called Cambrian explosion, when complex life rapidly diversified.
Prof Gernon added: "Today's escarpments in Africa, Brazil, India and Antarctica provide windows into the forces that shape continents over hundreds of millions of years.
"By comparing Grand Canyon's ancient history with active landscapes like the Great Escarpment of South Africa, we're able to see North America's most iconic geologic landmark in an entirely new light.
"Our findings could help geologists reinterpret other ancient continental interiors where similarly large gaps occur in records, offering a better understanding of how Earth's continents have changed over hundreds of millions of years."
